1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device for displaying colored patterns by use of a twisted nematic type liquid crystal cell.
2. Description of the Prior Art
It has been known in the art to utilize a liquid crystal material for display purposes. Particularly, a twisted nematic type liquid crystal cell has often been employed in liquid crystal display devices. The liquid crystal display devices employing a twisted nematic type liquid crystal cell have been used as digital display devices for electronic clocks or watches, electronic desk calculators, electronic computers, measuring instruments and so forth. A liquid crystal display device of this type is provided with a liquid crystal cell which comprises a pair of oppositely disposed transparent front and rear substrates bearing on the inside surface thereof a patterned conductive electrode layer, and a nematic-phase liquid crystal material having positive dielectric anisotropy filling the space therebetween. The space between the substrates, i.e., the thickness of the liquid crystal cell, is normally 1 to 50 microns. As the nematic-phase liquid crystal material, a material having positive dielectric anisotropy such as p-n-hexylbenzylidene-p'-cyanoaniline, is employed. The inside surfaces of the transparent substrates are rubbed at right angles to each other so as to effect a twisted nematic structure. Outside and adjacent the front and rear substrates are provided crossed linear polarizers to constitute a liquid crystal display assembly which transmits light therethrough when no electric field is applied across the liquid crystal cell. The transmittivity of collimated light through the twisted nematic type liquid crystal cell between crossed linear polarizers is decreased as a function of the strength of the electric field applied across the liquid crystal cell. In the absence of an electric field, an observer observing the device in the direction normal to the plane of the substrates and parallel to the normal optical axis will observe that the field of view is extinguished only if the direction of vibration (direction of linear polarization) of one linear polarizer is parallel to that of the other. It will be understood by those skilled in that art that the transmittivity of collimated light through the liquid crystal display assembly employing a twisted nematic type liquid crystal cell is increased as a function of the strength of the electric field applied across the liquid crystal cell when the linear polarizers disposed outside and adjacent the transparent substrates are oriented in parallel to each other. By rotating one polarizer by 90.degree. with respect to the other linear polarizer about the optical axis of the liquid crystal cell, the image-background combination of the displayed pattern is reversed from dark-on-light to light-on-dark or vice versa.
The pair of linear polarizers disposed adjacent the transparent substrates are achromatic (e.g. L-81-18 made by Sanritsu Denki K.K.), and accordingly, the image-background combination of the pattern displayed by the above described display devices is limited to dark-on-light or light-on-dark. However, since the sense of sight is more sensitive to change in color than change in liminosity, it is desired to make a color display, i.e., to change dark-on-light (or light-on-dark) image-background combination to colored-on-light or dark-on-colored (or colored-on-dark or light-on-colored) combination.
It has been known in the art to provide a liquid crystal display device which makes a colored display. One example of such a display device is disclosed in Japanese Utility Model Public Disclosure No. 27474/1974. In this color display device, a color filter is disposed in front of or behind the display assembly comprising a liquid crystal cell and a pair of linear polarizers, or between the cell and one of the linear polarizers to make the whole area of the display face colored.
In the above-described conventional color liquid crystal display devices for displaying colored patterns, the display face is colored in the color of the filter disposed therein with the liquid crystal cell becomes transparent, e.g. when the twisted nematic type liquid crystal cell is sandwiched between a pair of crossed polarizers and no electric field is applied thereacross, and the display face is darkened when the liquid crystal cell becomes opaque. Therefore, the image-background combination displayed thereby is limited to dark-on-colored or colored-on-dark. Therefore, the recognizability of the pattern displayed thereby is not enhanced except when the color of the filter is yellow which results in the highly recognizable contrast of black-and-yellow.
Further, in the above-described conventional color liquid crystal display devices for displaying colored patterns, the quantity of light transmitting through the liquid crystal display assembly including the color filter is decreased by absorption of light by the filter, which results in reduction in contrast of pattern displayed thereby.
In addition, the thickness of the liquid crystal device is increased by the provision of said color filter. Accordingly, the above-described color liquid crystal display devices including a color filter are particularly undesirable for electronic wrist watches which are required to be as thin as possible.